As I discussed in this thread, Validation function, PL/SQL is not being executed I am able to execute my PL/SQL validation function. But it seems that the list of parameters that I can pass to this function is limited to *:URL*. Would it be possible to use any other substitution string (http://docs.oracle.com/cd/E23903_01/doc/doc.41/e21674/concept_sub.htm#autoId2) like :APP_ID or :APP_PAGE_ID?

This forum is for Application Express Listener questions - your question is a general Apex function. You should get a better answer if you close this thread and re-open it in the Apex forum Oracle Application Express (APEX)

1) Is APEX_040200.is_allowed executable by apex_public_user? Also, you shouldn't be installing objects into the apex schema
2) Is there anything in the log file?
2.1) If nothing is in the log file, put it into debug and check the log file again.

1) Yes it is, and yes I know that I should not (most probably my DBA will kill me!), it was for testing purposes...
2) Tomcat: no it is not. When the apex context is deployed it shows its configuration, but nothing else. Oracle DB: I have no access, I will talk with the DBA.
3) org.apache.catalina.level=FINEST The same as above

Ok, great, I am starting to see the light. I have configured the log (standalone mode), following this post: http://cdivilly.wordpress.com/2013/03/08/configuring-logging-in-oracle-application-express-listener-2-0-1/

In the standard output I can see that the value of P_FLOW_ID is always null:

It seems that the only two possible binds for the parameters of the validation function are PROCNAME and URL. It is weird because if I set a wrong parameter like PROCNAM, I get the following exception in the logs...

java.sql.SQLException: The number of parameter names does not match the number of registered praremeters

Could this issue be caused by a Javascript problem? I have realized (decompilating a bit and taking a look at the Javascript) that and at the end of the day, the listener has to translate from f?p= syntax (f?p=4550:1:4595084190080) to "standar url syntax" (...p1=v1&p2=v2=...&pn=vn)...